Rotary electrical machines such as starters each comprise one or more electric fuse(s) or one or more circuit breaker(s) which are known hereinafter as a means for protection against excess current, which, in the case of a maximum predetermined current, respectively blow or open their contact in order to cut the electric supply of the rotary electrical machine after a certain time (time necessary respectively for the blowing or triggering).
The means for protection against excess currents can also be connected electrically upstream from the rotary electrical machine.
These means for protection against excess currents make it possible to prevent damage to the rotary electrical machine or its environment caused by the heat produced by the excess current. For example, in the case of a starter, the heat produced by a starter with excess current gives rise to heat which can damage the equipment in the engine compartment of the vehicle.
An example of an electrical machine 1 comprising a fuse 2 is represented in FIG. 1. The fuse 2 is fitted in series between a supply terminal 3 of the electrical machine 1 and the windings 4a, 4b, 4c, 4d of the stator of the rotary electrical machine 1.
In an electrical machine with direct current, it is known that the current increases strongly if the rotor is blocked in rotation. This increase in the current can reach the maximum predetermined current of the means for protection, which is equivalent to an excess current. In certain cases of normal functioning of a rotary electrical machine, the rotor can be blocked in rotation for several seconds (known as the blockage time hereinafter). This is the case in particular for the rotary electrical machines in starters, during normal use for starting of a thermal engine.
Consequently, the means for protection must be calibrated such that it cannot blow or open its contact for a predetermined time, whilst the current which passes through it is equal to, or greater than, the maximum predetermined current.
The predetermined time is determined by persons skilled in the art in a conventional manner according to the blockage time of the rotary electrical machine in normal use. For example, persons skilled in the art in the field of starters determine the predetermined time in a conventional manner according to the type of thermal engine to be started, the type of starter, and the most unfavourable environment selected by the vehicle manufacturers for starting of the thermal engine.
For example, for a starter, the calibration of the means for protection against excess currents, in this case the fuse, is selected such that the fuse blows when the rotor is blocked beyond the predetermined time necessary for starting of the thermal engine.
In other words, for a starter, the calibration of the means for protection against excess currents is selected firstly to blow or open its contact(s) when the time of a current passing through the rotary electrical machine, with its rotor blocked in rotation, which is equal to or greater than the maximum predetermined current, is longer than the predetermined time necessary for the starting of the thermal engine, but it must also not blow or open its contact(s) during the predetermined time when the current passing through it is equal to the current passing through the rotary electrical machine which has its rotor blocked in rotation.
However, when the machine is functioning unloaded (i.e. without a load or with a very low load), the unloaded speed of rotation of the rotor gives rise to heating of the electrical machine 1, caused in particular by the mechanical friction between the brushes and the collector. A low load means 0 to 25% of the of the nominal load. This heating increases the temperature of the environment in which the rotary electrical machine 1 is situated, and can damage the equipment or parts situated in this environment. For example, in the case of a starter connected to a thermal engine of a vehicle, which are situated in an engine compartment, when the starter rotates unloaded, i.e. when the thermal engine is disengaged from the rotor of the starter, which is the case in particular in an overspeed phase (when the pinion of the starter, driven by the thermal engine, rotates faster than the speed of the drive shaft which is rotated by the rotor of the starter), the temperature of the starter can damage the equipment which is situated in the engine compartment of the vehicle, in particular electronic components which are present increasingly in vehicles.
When the electrical machine is functioning unloaded, the current which passes through its fuse, and is also known as the no-load current (approximately 10% of the blockage current), as well as the increase in the temperature surrounding the means for protection against excess current, in this case the fuse 2, can be too low for the fuse 2 to be able to blow, even in the case of prolonged use of the starter in unloaded use.
A solution would thus consist of selecting a fuse, the calibre of which is selected such that it can blow both when the time of a no-load current passing through the fuse exceeds a first predetermined time, and when the time of a current passing through the rotary electrical machine with its rotor blocked exceeds a second predetermined time (the predetermined time for the blockage state), without the fuse blowing in the aforementioned cases. There are fuses with a volume which is far greater than a fuse of standard size (Le, a fuse with calibration selected in order to comply only with one of the two cases (blockage or unloaded)). This excessive volume can generate considerable transformation in the electrical machine or in the environment in which it is situated; for example, for a starter, the engine compartment of a vehicle.
In addition, for certain calibres required, which in particular is the case for certain vehicle starters, at present there is no fuse with a calibre which can comply with these two cases (case of blockage and unloaded).